1. Field of the Invention
The invention relates generally to the drilling of wells, such as those used for the production of oil and gas. More specifically, the invention relates to measuring downhole subsurface formation pressure.
2. Background Art
While drilling a borehole, the rock removed from the hole by the drill must be replaced with an equivalent weight to ensure stability of the formation. Drilling fluid, more commonly called drilling xe2x80x9cmud,xe2x80x9d is used to compensate for the weight loss of the removed rock by providing a stabilizing pressure in the well hole and hold back formation fluid pressure. Because there is a generally linear relationship between the hydrostatic pressure and the vertical depth of a column of fluid, the stabilizing pressure of the mud can be easily controlled by varying the density of the mud.
It is desirable to maintain the mud pressure at a level slightly higher than the the formation pressure to avoid problems in well development. If the mud weight is much greater than the formation pressure, a condition called mud over-balance occurs, and the mud will deeply invade into the formation. Such deep invasion can reduce the production capabilities of a well and could completely block any passage of fluid into the well from the formation. If the overbalance is great enough, you can fracture the well, causing xe2x80x98lost circulation.xe2x80x99 Conversely, if the mud weight is under-balanced, where the formation pressure is greater than the mud pressure, the well is susceptible to a blowout, resulting in an uncontrollable and unrecoverable loss of material from the well. If the formation pressure is known during an early stage of development, the well can be developed in such a way as to optimize well production.
Further, when the mud is over-balanced, the mud in the borehole will form a highly concentrated layer of solids at the borehole wall interface of the formation. This layer is called the xe2x80x9cmud cake.xe2x80x9d The thickness of the mud cake depends on, among other factors, the differential pressure between the formation and the borehole. By balancing the mud pressure with the formation pressure, the mud cake layer thickness is optimized, thereby reducing the chance that any well servicing or drilling tools will become stuck within the well.
FIG. 1A shows a top view of a borehole 11. When borehole 11 is filled with mud, the mud will form a mud cake layer 13. In a mud over-balanced situation, mud pressure is so high that mud will invade the formation 12, causing a skin-damage zone 14. In the skin-damage zone 14, the formation properties, including pressure, permeability, and porosity, are affected by the invading mud. FIG. 1B shows the same situation from a side view.
Methods for measuring formation pressure known in the art include removing the drill-pipe (xe2x80x9ctripping the wellxe2x80x9d) so that measuring instruments can be lowered into the open borehole. After these measurements are made, the drill-pipe is reinserted into the borehole so that drilling operations can continue. Because tripping the well in this manner is usually not done solely to allow for downhole measurements, formation pressure is not typically measured unless the drill-pipe is removed for another reason.
One technique for measuring formation pressure is called the draw-down or pre-test method. In this method, a formation tester tool is sent downhole to measure the formation pressure. The formation tester tool includes a donut-shaped rubber packer that is pushed against the borehole wall in order to isolate a small area of the formation face from the borehole pressure. Once in place, a hydraulically powered piston is moved within a test chamber in the tool, until the pressure in the small isolated area is significantly below the formation pressure. This pressure differential causes fluid to flow from the formation into the chamber. Over time, the pressure in the tool will stabilize to the formation pressure.
The pre-test method has several limitations. First, in low permeability formations, it can take several days for the pressure in the tool to converge to the formation pressure. Having the tool downhole for such an extended period of time can lead to tool sticking, making it difficult to remove the tool from the borehole. Also, large pressure imbalances can lead to packer failure and can tend to plug the tool with formation solids. Another problem is that the pre-test method uses large, heavy tools that require supplying hydraulic power to the tool while it is downhole. Finally, because of high stresses across the packer, the pre-test method does not work well in unconsolidated formations.
Another method for measuring formation pressure is described in U.S. Pat. No. 6,164,126, which is assigned to the assignee of the present invention. A probe is extended from a downhole tool into the formation. The probe extends through the mud cake and penetrates into the formation. Because the probe has a tapered shape, it creates a seal between the probe and the mud cake, and a packer is not required. The probe must penetrate the formation to a sufficient depth from the borehole so that it senses the formation pressure without substantial interference from the borehole fluids, that is, past the skin-damage zone. Unlike the pre-test, there is typically no pressure draw-down.
While the probe method overcomes some of the limitations of the pre-test method, it still has some limitations of its own. First, the probe must generally penetrate the formation past the skin-damage zone. By doing so, the probe itself may affect the pressure of the formation. When the probe is inserted, the displacement may cause the formation pressure to increase in the area of the probe. It is difficult to predict the amount of pressure increase because it will vary with the formation porosity and permeability. This increase typically diffuses or dissipates over time. Finally, when the probe is removed, it can leave a hole in the mud cake and the formation. This can allow the mud to invade the formation by flowing into the hole.
Recent advances in drilling fluid performance have made it possible to develop a well with substantially zero skin zone. A formation with no skin zone allows for the possibility of measuring the formation pressure with minimal penetration of a probe or sensor into the formation.
Another problem faced by previous devices is clogging. Typically, an opening in a probe may be blocked by rock particles from the formation, or completely covered by rock particles thereby sealing the opening and preventing a valid pressure measurement.
There remains a need to further develop techniques for evaluating formation properties. To this end, the present invention seeks to develop improvements in the testing process.
One aspect of the invention is a formation testing tool with a nozzle included therein. The nozzle is adapted to be moved between a retracted position and an extended position. In the extended position, the nozzle penetrates the mud cake and comes into pressure communication with the formation. In the extended position, the nozzle extends through the mud cake layer, creating a seal between the mud cake layer and a sealing surface on the exterior of the nozzle. A pressure sensor is operatively connected to the nozzle. Another aspect of the invention is a formation testing tool positionable in a wellbore having a sidewall. The tool comprises a nozzle and a tip. The nozzle is extendable from the tool into a mudcake layer lining the sidewall of the wellbore. The nozzle has a duct therethrough in pressure communication with a pressure sensor in the tool, and defines an outer surface adapted to sealingly engage the mudcake. The tip is at an end of the nozzle. The tip is adapted to restrict access to the duct whereby mudcake particles are prevented from entering the duct during formation testing.
Another aspect of the invention is a method for measuring formation pressure. The method according to the invention includes lowering the formation testing tool to a desired measuring position. The nozzle is then extended from the retracted position to the extended position, so that it penetrates the mud cake to the formation wall (rock face) in the borehole and the nozzle forms a seal with the mud cake. The formation pressure is communicated via an orifice in the tip of the nozzle, through the nozzle, and to a pressure sensor operatively connected to the nozzle.
Another aspect of the invention is a formation testing tool including a tool body adapted for movement through a wellbore. An actuator is disposed in the tool body and adapted to move a nozzle from a retracted position to an extended position. A nozzle in the extended position penetrates through a mud cake layer by an amount necessary to expose a tip of the nozzle to formation pressure. A tip is provided in an axial end of the nozzle. The tip has pores with a diameter smaller than a particle size in the mud cake layer. The nozzle having a passage therethrough in pressure communication with a pressure sensor in the tool. The passage is opened upon positioning of the nozzle tip. Another aspect of the invention relates to a method of testing a formation by lowering a formation testing tool to a first selected measuring position in a borehole, extending a nozzle through a mud cake layer on the sidewall of the borehole to form a seal between the mud cake layer and a sealing surface of the nozzle, positioning the tip of the nozzle to expose a passage in the nozzle to the formation pressure, and communicating the formation pressure through the nozzle to a pressure sensor. The nozzle has a tip at an end thereof and a passage therethrough. The tip may be porous or retractable to restrict access to the passage.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.